Phosphonous acids and their esters are valuable synthesis building blocks for preparing polymers and plastics. The phosphinic acids and phosphinic esters which are accessible from phosphonous acids and their esters can be used as comonomers in the preparation of polyesters. In this manner, low-flammability polyesters are obtained, which polyesters can be processed to form fibers, for example.
According to DE-A 21 00 779, dialkyl dialkylphosphinates are obtained by adding phosphonous esters to olefins using free-radical initiators.
DE 25 40 283 A1 describes the addition of phosphines to .alpha., .beta.-unsaturated carboxylic acids in the presence of aqueous hydrochloric acid and subsequent oxidation.
Phosphonous monoesters can be obtained by esterifying phosphonous acids. Ethylene oxide, for example, can be used for the esterification (Houben-Weyl, Volume 12/1, p. 320).
In addition, phosphonous monoesters are accessible from phosphonous diesters by saponification or from phosphonous dihalides by reaction with alcohols (Houben-Weyl, Volume 12/1, p. 320). Usually, the phosphonous diesters are prepared from phosphonous dihalides.
The abovementioned phosphonous dihalides, eg. methyldichlorophosphine, which can be used as starting materials for other syntheses, have themselves been prepared to date in a complex synthesis from phosphorus trihalides and alkyl halides in the presence of aluminum chloride (Houben-Weyl, Volume 12/1, p. 306). The reaction is highly exothermic and can be controlled only with difficulty industrially. In addition, various byproducts are formed which are, as are also some of the abovementioned starting products, toxic and/or corrosive, and therefore highly undesirable.
The abovementioned processes are, in addition, complex and cannot be carried out on an industrial scale without difficulty.
There is therefore a requirement for a process for preparing functional phosphonous esters, which process can be carried out in a simple manner and in which uniform products are obtained in high yield. A process of this type should also be clearly superior environmentally to those known hitherto.